The invention refers to a method for exposing a layout comprising multiple layers on a wafer, in which at least one layer is exposed photolithographically and subsequently at least one further layer is exposed with an electron beam, and the wafer is to be aligned in defined fashion with respect to the electron beam exposure system in terms of the previously photolithographically exposed layer.
Lithographic methods in the manufacture of wafers often require successive exposure of multiple layers located one above another or areas located next to one another. It is important in this context that the successively exposed layers be positioned exactly with respect to one another, and that the areas located next to one another adjoin one another in precisely fitting fashion. Alignment marks are usually provided on the wafers for this purpose.
Both photolithographic methods and electron beam lithographic methods are used to expose wafers. Electron beam lithographic methods are characterized by higher resolution than the photolithographic methods. The production rates of steppers that operate on the photolithographic principle cannot, however, be achieved with conventional electron beam writing machines.
If high resolution and high productivity are required simultaneously, so-called xe2x80x9cmix and matchxe2x80x9d exposure systems offer a way out in many cases. This means that steppers perform most of the lithography and expose almost all the layers of a layout. Electron beam lithography is employed only for layers with a high resolution. Either critical layers (gate layers) with a low packing density are exposed in their entirety, or those parts of a layout that require high resolution are filtered out (intra-level mix and match), and only they are delineated by electron beam lithography. In both cases, measurement of the alignment marks is of particular importance.
These alignment marks are, however, usually relatively small, so that they cannot readily be located. In practice, in fact, several steps are necessary to do so. In one known method, after a coarse alignment of the wafer, for example by orientation at its geometrical edges, an intermediate step using first marks (hereinafter also called xe2x80x9cglobal alignment marksxe2x80x9d) is performed before the actual fine alignment is accomplished based on the alignment marks. One of the first important steps in the course of an exposure method is therefore global alignment, i.e. orienting the wafer using the first marks. Their size depends on the accuracy achievable during coarse alignment. First marks with an extension of typically 0.5 mm to 1 mm are usually used as search structures for global alignment, to allow the much finer alignment marks to be found later on.
In practice, for this purpose the position of at least one first mark is determined in the intermediate step. The position of the alignment marks can then be better determined with the aid of a coordinate transformation, so that they can be found in a small search field and, for the actual alignment of an image field of a chip on the wafer, aligned with respect to the particular lithography device.
Marks in the form of crosses with an extension of 1 to 2 mm are often used in electron beam lithography as global alignment marks. The evaluation algorithms usually require that the crosses be located in an undisturbed environment, i.e. that no other marks or similar structures be located in the immediate vicinity of the crosses, since otherwise the evaluation algorithm is disrupted.
To find these marks, in the known methods scans are first performed in a first direction with the electron beam. For this purpose, the electrons back-scattered from the wafer are sensed. An evaluation algorithm determines whether the mark was encountered in the course of the respective scanning operation. If so, its position is determined. An analogous procedure is then used in a second direction, thereby ultimately yielding the position of the mark.
The complete global alignment process measures the positions of multiple first marks and calculates the necessary transformations for a relative motion between the wafer and the lithography system, for example of coordinates of a stage on which the wafer is placed in terms of translation, rotation, and scaling, in order to find the alignment marks.
Proceeding therefrom, it is the object of the invention to improve a method of the kind cited initially in such a way that alignment of the wafers is simplified, and a higher throughput of wafer exposures per unit time is thereby attainable.
This object is achieved in that before electron beam exposure begins, an alignment of the wafer (global alignment) with respect to the electron beam exposure system is performed on the basis of structural features from the layer that was previously exposed photolithographically.
As a result, the marks that in the existing art needed to be provided on the wafer specifically for the electron beam exposure are no longer necessary. In other words, the method according to the present invention is advantageously no longer tied to special mark geometries; instead, global alignment can be based on a wide variety of structural features that can be selected almost at random from a layer that has already been exposed. The wafer can thus be aligned on any desired structure on the wafer in the electron beam exposure system, and the wafer does not require any separate structures that serve only for alignment in the electron beam exposure system.
In addition, advantageously, it is possible to use for alignment not only selected structural features from another layer, but also marks that originally were applied onto the wafer by no means specifically for electron beam exposure. Whereas different mark geometries hitherto had to be used for photolithographic methods and electron beam lithography methods, in this particular application of the method according to the present invention it is now possible to use common global alignment marks for alignment for both exposure methods.
In addition, it is possible to use the devices usually present in any case in the context of electron beam lithography or so-called mix and match lithography. This applies in particular to the deflection device for the electron beam, the devices for detecting backscattered electrons and for analog-digital conversion of the signal curves sensed in that context, and the devices for storing and processing data.
In a preferred embodiment of the invention, provision is made for first positioning the wafer coarsely with respect to the electron beam exposure system in such a way that a surface area of the previously photolithographically exposed layer that contains the selected structural features is located in the deflection region of the electron beam; then scanning the surface area in the X and Y direction in spot fashion with the electron beam, detecting the radiation backscattered from the individual spots, and obtaining from the intensities of the radiation backscattered from the individual spots an actual data set that corresponds to a raster image of that surface area with the wafer in the actual position; comparing the actual data set to a stored reference data set that corresponds to the raster image (template) of the same surface area with the wafer in the reference position; deriving from the deviation between the two data sets information as to the deviation between the actual position and the reference position; on the basis of that information causing a positional change of the wafer in order to bring the actual position closer to the reference position; then once again obtaining an actual data set and comparing it to the reference data set; and repeating the obtaining of an actual data set and the comparison with the reference data set, as applicable, until no further deviation is ascertained.
It is thereby possible to perform compensation for a positional deviation based on determination of the position of the selected structural features, so that the method according to the present invention can easily be used for automatic alignment of a wafer.
The signal curves obtained during spot scanning of the surface area are preferably digitized. The raster image is then also stored in digital form, thereby making possible easy storage and further processing.
As already explained above, the method according to the present invention is not limited to the use of special marks or structures as the basis for alignment. In an advantageous embodiment of the invention, it is thus possible to obtain the reference data set by means of a wafer that has already been exposed, the wafer and the electron beam exposure system first being displaced relative to one another until the selected structural features are located in the reference position, and a data set corresponding to the raster image of the surface area having those structural features then being acquired and stored. It is thereby possible, in principle, to use any desired characteristic structural features as recognition patterns. An electron beam exposure system in which a software program operating according to the method is implemented can thus be utilized very flexibly, allowing a wide variety of structural features to be selected as global alignment marks.
As an alternative to this, the reference data set can also be taken from electronically available structure data of the layer that is to be exposed photolithographically, and stored as the recognition pattern. It is conceivable, for example, to generate the corresponding geometry with a graphics program and to translate it into the format of the raster image.
It is also possible, however, to continue to use as the characteristic structural features, for example, the marks hitherto used for alignment in an electron beam device or even other geometric structures specially applied onto the wafers for that purpose.
Particularly advantageous, however, is the utilization according to the present invention of a characteristic structure area from a previously exposed layer, since in this case the application of special marks can be completely dispensed with. xe2x80x9cCharacteristicxe2x80x9d means in this context that at least the region in the immediate vicinity of the structure area is free of similar structures.
The structure area examined for determining the position of the wafer should preferably have a size of up to 2000 xcexcmxc3x972000 xcexcm. Referred to an area of this kind, scanning should be accomplished with between 20xc3x9720 and 1000xc3x971000 spots.
In a further advantageous embodiment, during raster scanning of the surface area to acquire the raster image with the electron beam, a spot chain is applied onto the substrate. In this context, the step size of the electron beam is in the range from 20 nm to 5 xcexcm. The raster images obtained by way of the backscattered electrons ensure on the one hand a sufficient data volume for finding the selected structural features in the raster image using image data processing methods, and on the other hand a relatively short calculation time so that the structure being sought can be located quickly. Times of less than 60 seconds per raster image for finding and determining any positional deviation are possible.
During raster scanning, the surface area can also be divided into micro-fields that are scanned individually and assembled into an overall image. The division is preferably made into micro-fields having a maximum size of 60xc3x9760 spots each. In the context of storage of the image data in digital form, this is possible without major problems. Advantages in terms of image acquisition are thereby obtained, in particular in the context of hierarchically structured two-channel electron beam deflection systems, since partial images corresponding to the micro-fields can be obtained first using the lower-order sub-reflection system. This is favorable in terms of a high image acquisition rate.
A suitable hardware system is described in xe2x80x9cEvaluation of Fine Pattern Definition with Electron-Beam Direct Writing Lithography,xe2x80x9d SPIE, 3997, pp. 646 ff, 2000. The content of this publication is incorporated into the present Application.
Global alignment using the method according to the present invention can be followed by fine alignment of the wafer on the basis of alignment marks present on the wafer. This involves the use of method steps that are already usual in the existing art. Compensation for a positional deviation with respect to the reference position can be integrated, by way of a corresponding correction of the approach motion for the search field, into the approach to a search field in which an alignment mark is suspected to be present. Fine alignment of the relevant image field or chip, and thus of the wafer, with respect to the electron beam lithography system is then performed on the basis of the alignment mark.